unwrap cube with circle
The problem you're having are three N-Gons. N-Gons are faces with more than 4 verts. So you need to change the N-Gons to tris or quads. To triangulate a face, just press SPACE and type triangulate, you will then see the option "triangulate faces".

The first picture shows my unwrap with N-Gons, the second one is with triangles. 




However, the right way of doing it (if you want to maybe use sub-d or something) would be like this. You the only would have to deal with the edges going over the cube.: